Cargo aircraft customarily load items of freight in “single row”, “side by side” or “triple row” configuration. That is, the freight items are stored next to one another in arrays oriented in the long direction of the cargo hold within the aircraft fuselage, such that the arrays can comprise one, two or three rows. There are also variants of this scheme, in particular “multiple choice systems” that allow two or three combinations simultaneously. The single-row array is customarily always disposed down the middle along the center line of the aircraft, because in this way it is possible not only to solve the weight/balance problem, i.e. to position the center of gravity as desired, but also to place the freight items where the greatest vertical space is available.
In B 747 freighters so far only the side-by-side configuration has been possible. When items of freight are to be loaded along the center line of the aircraft, it has previously been necessary to take the elaborate measure of lashing them down to the floor with retaining belts fastened to the seat rails. In the case of heavy loads, such as engine pallets, this operation can take up to several hours for each pallet. Furthermore, the region around these freight items (which in such cases are most often pallets) is no longer usable because it is occupied by the retaining belts, which run downward at an angle.
A major problem here is that although adapters and structures especially designed for retaining such equipment could be installed in the floor of the cargo hold, this would require substantial structural alterations and be enormously expensive.
One possible means of solving this problem when voluminous and heavy loads are to be stored is provided by a system along the lines of a spider web: that is, a network of extruded profiles distributed over the floor of the cargo hold, between the parts of the cargo-storage system that are already present, and installed there as a means of fixing these loads in position. However, such a system causes a blockage in the region around the freight items that are to be secured, so that on one hand the space next to the region into which such an item is loaded cannot be used for anything else, and furthermore the “spider-web system” must be disassembled in order to fill or empty the remaining storage space in the aircraft.
Particularly important parts of an aircraft (especially the B 747) are the region ahead of the side cargo-loading door and behind the door in the nose, and, in all aircraft, the storage space above the wing box, because this space can hold almost twice the weight per item of freight as the rest of the cargo area. For the above reasons, i.e. in particular for reasons of the limitations on installation of retaining devices imposed by the builders of the aircraft, the fixation of freight items in these regions, in particular along the center line, is necessarily uneconomical.